1. Field of the Invention
The present invention relates to the stabilization of laser oscillator output.
2. Description of the Background Art
FIG. 6A is a perspective view illustrating the arrangement of a known laser oscillator disclosed in Japanese Laid-Open Patent Publication No. 254683 of 1985. In this drawing, the numeral 10 indicates an enclosure for enclosing laser medium gas (hereinafter referred to as the "laser gas"), 4 denotes a pair of discharge electrodes, 8 represents a heat exchanger, 6 designates a blower, 32 indicates a partial reflector, 26, 28 and 30 denote total reflectors, 12a represents a first laser beam reflecting structure which includes the partial reflector 32 and the total reflector 28, 12b indicates a second laser beam reflecting structure which includes the total reflectors 30 and 26, and 2 designates a laser beam. An exciting area 18 is created in the space between the pair of electrodes 4.
FIG. 7 is a sectional view of the first laser beam reflecting structure 12a, wherein 56 denotes an aperture plate disposed immediately in front of the partial reflector 32 and the total reflector 28 for shaping the beam. 36 indicates an optical base for holding the first laser beam reflecting structure 12a, 44 represents connecting bars for coupling the enclosure 10 and the optical base 36 of the second laser reflecting structure 12b, 54 designates bellows fitted to keep the enclosure 10 and the optical base 36 vacuum and air-tight, 42 represents an adjustment plate fitted with the partial reflector 32 for adjusting the angle of the partial reflector 32 and having an inner surface 46, 40 denotes angle adjustment screws, and 48, 58, 58a and 58b indicate duct structures. 41 indicates springs that serve to provide a bias for adjustment of the plate 42. A beam passing aperture fitting 45A is formed in adjustment plate 42 and defines a beam passing hole 45.
FIG. 8 shows an expanded view of the mounting section of the partial reflector 32 disclosed in Japanese Laid-Open Patent Publication No. 254683 of 1985. In this drawing, 3 indicates a laser medium gas flow, and 4A denotes a hole bored in the optical base 36. 5 represents a space formed between the surface 46 of adjustment plate 42 and the beam passing aperture fitting 45A. The end of duct cylinder portion 58a and the end of beam passing aperture fitting 45 A are separated by a distance .delta. to permit passage of the laser gas. The arrangement of FIG. 8 is a direct water-cooled type laser which directly cools the partial reflector 32 with water. It should be noted that this arrangement is not often used at the present time.
FIG. 9 shows an expanded view of the mounting section of the partial reflector 32 in an indirect water-cooled type laser, which is currently a favored design. As is clear from a comparison between FIG. 8 and FIG. 9, the direct water-cooled type in FIG. 8 has a larger number of parts in the mirror supporting portion.
Operation will now be described with reference to FIGS. 6A and 6B. The enclosure 10 contains a pair of discharge electrodes 4 for generating electrical discharge therebetween and exciting the laser medium gas, the blower 6 for circulating the laser medium gas, and the heat exchanger 8 for cooling the laser medium gas. The laser medium gas passes between the pair of discharge electrodes 4 and is excited therein to be laser oscillatable, then enters the heat exchanger 8 and is cooled therein, and passes through the blower 6. The circulation of the laser medium gas in the direction of arrow A is the "main flow".
Three Z-shaped optical paths, created by the resonator mirrors consisting of the partial reflector 32 and the total reflectors 26, 28, 30 placed in the longitudinal direction of the enclosure 10, pass through a resonator excitation area 18 where the laser medium gas has been excited by electrical discharge.
The laser beam reflected by the total reflector 26 passes along a first optical axis 20, reaches the total reflector 28, and is reflected thereby. The laser beam reflected by the total reflector 28, which is inclined slightly downwardly, passes along a second optical axis 22, which is inclined slightly downwardly from the first optical axis 20, reaches the total reflector 30, and is reflected thereby. The laser beam reflected by the total reflector 30, which is inclined slightly upwardly, passes along a third optical axis 24 in parallel with the first optical axis 20 and reaches the partial reflector 32. Part of the laser beam having reached the partial reflector 32 is output intact to the exterior as beam 2 and the remainder thereof returns to the total reflector 26 along a backward route. In general, approximately 30% of the beam incident on the partial reflector is passed as beam 2 and approximately 70% of the incident beam is reflected back. This process is repeated thereafter. As is well known, the laser beam is amplified while it repeats passing the resonator excitation area 18 as described above.
The angle adjustment mechanism of the partial reflector 32 in FIG. 7 will now be described. The partial reflector 32 is installed on the adjustment plate 42. By adjusting the angle adjustment screws 40, the angle of the adjustment plate 42 is adjusted, whereby the angle adjustment of the partial reflector 32 is made.
The optical base 36 is provided with a hole 4A through which the laser beam passes. Extending within the volume defined by the base 36, the enclosure 10 and the bellows 54 is a duct member 58. The duct 58 member consists of pipes 58a and 58b and a bottomless box-shaped connecting member 59. Inserted into the hole 4A is a cylindrical portion 58a of the duct member 58 which serves to guide a portion of the laser medium gas as a "side flow". The front end of the cylindrical portion 58a extends toward the partial reflector 32 and is proximate the exposed end of beam passing aperture fitting 45A. This structure causes the side flow portion of the laser gas to flow to a location adjacent the partial reflector 32. The other end 58b of the duct member 58 is inserted in the coupling duct 48, which is fixed to the enclosure 10.
The "side flow" is developed by the structure of the device. Specifically, a space between the discharge electrodes 4, where the main flow A of the laser gas flows at high speed, is in a negative pressure state. By contrast, the coupling duct 48 is fixed to a high-pressure portion (not shown) where there is no gas main flow from the blower 6 in the enclosure 10. Hence, the laser gas from the high pressure area is sucked into the duct 48 as indicated by "a" in FIG. 7 and is sucked out of duct 58 toward the discharge electrodes 4 as indicated by "b".
The flow path of the "side flow" provides the following feature. When there is no side flow, the laser gas in the optical path of the laser beam 2 stays adjacent the mirror 32. The laser gas is a laser beam absorbent medium and changes some of the energy of the laser beam that passes through the laser gas into heat. This raises the temperature of the laser gas that stays adjacent to the mirror 32. Further, the laser beam absorbing ratio of the laser gas increases with the temperature rise, and greater heat generation starts, thereby further raising the temperature. Therefore, it is difficult to maintain stable laser output and the desired performance cannot be secured.
For the above reason, the side flow is required to prevent the temperature of the laser gas adjacent the mirror from rising. FIG. 10 indicates the flow rate of the laser gas flowing inside the duct cylindrical portion 58a and the temperature of the laser gas with respect to a distance from the partial reflector 32. When the laser gas is flowing outside of the duct cylindrical portion 58a, the temperature does not rise since the laser beam is not absorbed. However, when the laser gas enters the duct cylindrical portion 58a and proceeds toward the resonator area, the temperature rises gradually due to the absorption of the laser beam because the laser gas flows in the laser beam path. The laser gas must therefore be forced to be circulated by the side flow in a manner which does not cause the "critical temperature" (i.e., one at which the maximum temperature Tmax of the laser gas existing in the optical path of the laser beam 2 cannot maintain a stable laser output) to be exceeded. The temperature at which the stable laser output cannot be maintained depends on the output and mode of the laser oscillator.
As described above, the side flow is provided to maintain stable laser output. However, it is impossible to avoid the entry of pollutants, such as dust trapped at the time of assembling or maintenance and/or an outgas component generated by the enclosure, into the laser gas even slightly. When such pollutants are carried by the side flow and impinge on the surface of the partial reflector 32, some become attached to the surface of the partial reflector 32 by the laser beam. The resultant film increases the absorbing ratio of the partial reflector 32 and causes the laser beam mode to change, thereby degrading the laser beam quality.
To solve such a problem, Japanese Laid-Open Patent Publication No. 254683 of 1985 presented the arrangement as shown in FIGS. 8 and 9; but this design was found to be insufficient. In the arrangement of FIGS. 8 and 9, the dynamic pressure of the laser gas flowing outside of the duct cylindrical portion 58a is converted into a static pressure by the space 5, which is found along the path of flow. The laser gas having a higher static pressure is directed toward the optical path of the laser beam. However, since there is no means for precisely controlling the direction of the laser gas, the side flow does not have a streamline characteristic that is directed only away from the surface of the partial reflector 32. Instead, a portion of the gas is diverted at the end of beam passing aperture fitting 45A and a spiral flow of the diverted gas is produced at the surface of the partial reflector 32. This spiral flow of the laser gas still carries the pollutants up to the surface of the partial reflector 32, causing the partial reflector 32 to be contaminated by the pollutants. When a distance between the end of the beam passing aperture fitting 45A and the surface of the partial reflector 32 is increased to prevent the spiral flow from reaching the partial reflector 32, the laser gas adjacent the partial reflector 32 will remain, thereby causing the temperature of that area to rise and generating output instability.
In the conventional laser oscillator arranged as described above, contamination occurring due to dust and the like in the gas increases the absorbing ratio of the partial reflector 32, reducing the laser beam quality.